"Too tall, too skinny, too orange" - that's Domhnall Gleeson's character Tim Lake's withering assessment of his own sex appeal at the start of About Time. It also sounds like something a Hollywood executive would say about the Dublin actor trying on the Hugh Grant crown for size in rom-com supremo Richard Curtis' (Four Weddings and a Funeral, Notting Hill) latest - and reportedly last - film. But both the big shot and Tim have got it wrong: in this hopeless-to-hero story you're cheering on the underdog actor just as much as the misfit he's playing.

Plenty of smarts but next-to-no slick, Tim has lived a pretty sheltered life with his preternaturally laidback dad (Nighy), stoical mum (Duncan) and eccentric uncle D (Cordery) beside the sea in Cornwall. But as Tim sees out 2004 and faces up to another year alone, he gets the biggest of January 1 surprises; not a kiss from a dream girl, but the sharing of a great family secret.

In one of their frank-but-chilled, father-son chats, Tim's dad tells him that, like all the other men in the family, he can travel through time - all he has to do is jump into a wardrobe and, after a bit of metaphysical mumbo jumbo, he's back where he wants to be. Now, as Lake Senior tells it, Tim can't "kill Hitler or shag Helen of Troy", but he will have the power to change the past in his own life - for better or for worse. There are a lot of déjà vu dilemmas ahead, but first to right that New Year's Eve party...

Tim's not the only one to have been here before, because those of us whose seen-it-a-hundred-times-will-watch-it-again movies include Four Weddings and a Funeral, Notting Hill or Love Actually will find plenty of comfort blanket familiarity in About Time. The surprise is that, in between the Curtis calamities, characters and ideals, there are plenty of lump-in-throat moments and heavy questions of the awake-at-4am variety. Don't let the jaunty TV trailer fool you into thinking this is all fun from start to finish; it's basically two movies for the price of one. That gear-change is somewhat jarring, but stick with the story because the payoff is worth it - inside the cinema and out.

While there has always been grief or a yearning for what might have been in Curtis' stories - the sudden death in Four Weddings..., the wheelchair-bound friend in Notting Hill, the bereaved child, doomed office romance and affair in Love Actually - here the sadness is more pronounced and thought-provoking, as if the writer-director is facing up to his own mortality. In asking viewers to think about theirs he has chosen the perfect leading man in Gleeson, who carries both the drama and comedy with equal confidence and makes the multiplex his own. Had McAdams' character been equally developed, we would've been talking about a great screen romance; instead the most touching aspect of the film turns out to be the relationship between father and son. It would be good if plenty of both were in the audience together - along with the mothers and daughters and all those couples.

Whatever our circumstances and achievements, one thing that unites us all is the belief that we didn't make the most of the time we were given, or didn't spend enough of it with the right people.

There's no need for such soul-searching about the two hours spent here.